Digital waveform synthesisers are known. Such digital waveform synthesisers comprise a direct digital frequency synthesiser which produces a synthesised output signal of a selectable frequency which is determined by the value of a frequency control digital word applied to a frequency control input of the direct digital synthesiser. By varying the value of the frequency control digital word, the frequency of the synthesised output signal may be varied. Typically, the direct digital synthesiser is clocked by a system clock signal which may be internally generated or externally generated, and comprises a modulo-M accumulator which on respective clock cycles repeatedly accumulates the value of the frequency control digital word for producing digital words which are representative of the phase of the synthesised output signal. A digital signal processing circuit converts the digital words which are indicative of the phase of the synthesised output signal from the accumulator to phase dependent digital words to produce a digital representation of the synthesised output signal. A digital-to-analogue converter converts the digital words which are representative of the synthesised output signal to an analogue representation thereof. Such digital waveform synthesisers are relatively versatile in producing synthesised output signals of selectable waveform and selectable frequency, and in particular for producing synthesised waveforms of selectable frequencies, since the frequency can be readily selected by selecting an appropriate value of the frequency control digital word. However, there is no easy way of operating such digital waveform synthesisers for locking the synthesised output signal produced by such synthesisers in a phase relationship with an externally generated input signal, for example, phase locking the synthesised output signal to the phase, and also the frequency of an externally generated signal.
The present invention is directed towards addressing this problem and providing a digital frequency synthesiser for producing a synthesised output signal in a phase relationship with an externally generated signal, and the invention is also directed towards providing a method for locking a synthesised output signal of a digital waveform synthesiser in a phase relationship with an input signal. The invention is also directed to a method for operating a digital waveform synthesiser and to a digital waveform synthesiser for producing a synthesised output signal waveform of a frequency for applying to an external circuit for producing a selectable response from the external circuit.